1. Field of the Invention
This invention relates to an improved process for the production of linear low density copolymers of ethylene and a minor amount of an olefin containing at least 4 carbon atoms (LLDPE).
2. Description of Related Art
The following information is disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
U.S. Pat. No. 4,405,774 issued Sep. 20, 1983 to Miwa et al., discloses LLDPE having improved transparency prepared by copolymerizing ethylene with an xcex1-olefin containing 4 to 10 carbon atoms utilizing a catalyst comprising an organometallic compound such as a trialkylaluminum and a contact product of a magnesium compound and a titanium compound. The copolymerization may be carried out in at least one continuous stirred reactor or at least one continuous tubular reactor.
U.S. Pat. No. 4,530,983 issued Jul. 23, 1985 to Matsuura et al., discloses the preparation of LLDPE by the copolymerization of ethylene and an xcex1-olefin having 3 to 14 carbon atoms in the presence of a catalyst comprising A) a solid composition prepared by mixing and pulverizing a magnesium halide, titanium trichloride, and an electron donor; and B) an organoaluminum compound containing a halogen.
U.S. Pat. No. 3,509,115 issued Apr. 26, 1970 to French, describes the production of ethylene-vinyl acetate copolymers at elevated temperatures and pressures wherein the product is subjected to a high pressure recovery step to remove unreacted ethylene, and the temperature is reduced during the recovery step by adding ethylene to the product stream prior to the recovery step to reduce acetic acid odor in the copolymer.
U.S. Pat. No. 4,215,207 issued Jul. 29, 1980 to Durand et al., describes a high pressure process for the polymerization or copolymerization of ethylene utilizing a separator operating at a substantially lower pressure than the reactor wherein the temperature of the separator is reduced by the injection of monomer into the reactor effluent at a pressure below that of the separator prior to its entry into the separator.
U.S. Pat. No. 4,558,105 issued Dec. 10, 1985 to Carrick et al., discloses the copolymerization of ethylene with a higher 1-olefin in the presence of an inert gaseous ratio modifier such as ethane for the purpose of avoiding liquid condensation and controlling polymer density.
Japanese published patent application (Kokai) JP SHO 61-276,804 [86-276,804] published Dec. 6, 1986 [abstracted in C.A. 106:157041u (1987)], discloses the production of LLDPE by copolymerizing ethylene and an xcex1-olefin in the presence of a catalyst prepared by prepolymerizing the xcex1- olefin with a solid component containing trivalent Ti, Mg and Cl, an organoaluminum compound, and an electron donor having Cxe2x80x94O or Cxe2x80x94N bonds.
Linear low density polyethylenes (LLDPE), which are generally linear copolymers of ethylene and a minor amount of an 1-olefin, i.e., a 1-alkene, containing at least 4 carbon atoms, may be prepared by contacting the comonomers with a Ziegler catalyst at a relatively high pressure e.g., 500 to 3000 bars, in a tubular or continuous stirred reactor. The effluent from the reactor is then passed to a xe2x80x9chigh pressure separatorxe2x80x9d (HPS) in which the polymer present in a dense phase is separated from the unreacted monomers present in a light phase. The separation is carried out at a pressure which is still high although somewhat lower than the reactor pressure.
Because the ethylene tends to polymerize much more readily than the 1-olefin comonomer, the concentration of unpolymerized 1-olefin in the HPS is normally significantly higher than that in the polymer (in polymerized form) or in the reactor feed mixture. Moreover, such concentration increases as the percent conversion of monomers is raised. An undesirable effect of this is that the polymer tends to become more soluble in the light phase containing the unreacted monomers as the percentage of 1-olefin in this phase increases with the percent conversion. Thus, in any particular system of LLDPE production as described, there is a practical maximum of percent conversion of monomers above which the percentage of unreacted 1-olefin in the light phase normally becomes high enough to cause sufficient polymer to dissolve in the light phase so as to give rise to plugging of the equipment downstream of the separator. Furthermore, the increased content of dissolved polymer in the light phase cannot be easily recovered and thus may significantly reduce the yield of polymer from the process.
In accordance with this invention, LLDPE is produced by contacting ethylene and a minor amount of a 1-olefin containing more than 2 carbon atoms with a Ziegler catalyst in a polymerization reaction zone at elevated temperature and pressure, contacting the polymerization reaction mixture containing polymer and unreacted monomers with a fresh supply of ethylene in a high pressure separation zone (HPS) operating at a lower pressure than the reaction zone such that the contents of the separation zone separates into a dense phase containing a major proportion of polymer, and a light phase containing a major proportion of unreacted monomers, and separately withdrawing the two phases from the separation zone. The polymer-containing dense phase is then subsequently treated to obtain a purified polymer product, and at least a portion of the unreacted monomers in the light phase is generally re-used in the reaction with or without some intermediate treatment or purification.
An intended result of the injection of a fresh supply of ethylene into the reaction mixture for separation in the HPS is that it reduces the concentration of 1-olefin in the light phase present in the HPS, because of the dilution effect of the added ethylene. This in turn reduces the solubility of polymer in the light phase thus increasing the yield and minimizing the tendency of dissolved polymer to plug the equipment intended to handle such light phase downstream of the HPS. Such reduced plugging tendency allows for the possibility of running the reaction at a higher monomer conversion rate before the concentration of polymer the light phase becomes high enough to cause disruptive plugging.